The present invention relates to an optical information recording/reproducing device for recording/reproducing information on/from an optical disk using a laser beam, and more particularly relates to an optical information recording/reproducing device for recording/reproducing information on/from both land sections and groove sections of the optical disk.
An optical disk device as an example of an optical information recording/reproducing device performs recording/reproducing of information in the following manner. That is, a light beam emitted from a semiconductor laser (light source) is converged by an objective lens (converging means) to form a light spot, and a tracking of the resulting light spot is performed along tracks formed on the optical disk, whereby information is recorded or reproduced based on the resulting signal.
To realize the described recording/reproducing operations, on the optical disk for recording thereon and reproducing therefrom information, tracks (guide grooves) are formed beforehand so that a tracking of the light spot can be performed along the tracks. Hereinafter, the groove sections where the tracks are formed are simply referred to as the groove sections, and the regions between adjacent grooves are referred to as land sections.
In the conventional optical disk devices, recording/reproducing of information are performed with respect to either the land sections or the groove sections of the optical disk. In recent years, however, the method for recording information both on the groove sections and the land sections on the optical disk has been proposed, which realizes an improved recording density of twice as high as that of the conventional optical disks.
However, in the described optical disk devices designed for recording/reproducing information on/from both the groove sections and the land sections on the optical disk, a problem arises in that an interference occurs as a tracking error signal is transferred to a focus error signal. This is known as a track interference, or a crosstalk between error signals, and hereinafter referred to as a crosstalk between error signals.
Due to the described crosstalk between error signals, a problem arises in that an optimal focus offset amount differs between (1) when a tracking is carried out with respect to the groove sections with the light spot resulting from converging light emitted from the semiconductor laser by the objective lens and (2) when a tracking is carried out with respect to the land sections with the light spot. For this difference in optimal focus offset amount, if the same servo control amount is used for both cases, an optimal focal position cannot be obtained, resulting in the problem that optimal recording/reproducing cannot be performed.
The reason why a difference in focus offset amount exists between when tracking the land sections and when tracking the groove sections will be explained in reference to FIG. 10.
FIG. 10 shows the focus error signal and the tracking error signal obtained from the optical pickup and these servo error signals are shown in the state where only the focus servo is set ON. The servo error signals (focus error signal and tracking error signal) are shown so as to correspond to error signals at respective positions of the spot as converged on the optical disk 66 by the objective lens (groove sections 66a and land sections 66b shown in the figure) of the optical disk 66.
As shown in the FIG. 10, generally, the focus error signal is affected by the tracks on an optical disk 66, and has the same period as the tracking error signal, but has a different phase from that of the tracking error signal. This can be observed when the frequency band fF of the focus servo is smaller than the track cross frequency fTC generated due to the eccentricity of the track.
The described deviations in the focus error signal is known as a crosstalk between error signals. The crosstalk occurs by the following mechanism. On the photodetector which generates a servo error signal, a reflected light from the optical disk is affected by aberrations of the optical components of the optical pickup, particularly the objective lens. As a result, an asymmetrical property is attributed to the reflected light, and thus tracking error signal leaks into the focus error signal, thereby generating a crosstalk between the error signals.
According to the servo control of the optical disk device, generally, the tracking servo is set ON after setting ON the focus servo. Therefore, as can be seen from the servo error signal shown in FIG. 10, by the effect of the crosstalk between error signals, when a tracking of the land sections 66b is performed with the light spot, the focal point is L as indicated in FIG. 10. On the other hand, when a tracking of the groove sections 66a is performed with the light spot, the focal point is G as indicated in FIG. 10.
As described, the focal point differs between when tracking the land sections 66b and when tracking the groove sections 66a. Therefore, if the focus servo is carried out with the same amount of servo control for both the groove sections 66a and the land sections 66b, a deviation in the direction of an optical axis (focus offset) occurs as indicated by 1+g, and an optimal focus offset amount differs.
Accordingly, the inventors of the present application disclose in the Japanese Unexamined Patent Publication No. 180429/1996 (Tokukaihei 8-190429) (U.S. patent application Ser. No. 08/539,523) the structure of switching the amount of servo control of the focus servo between when tracking the groove sections and when tracking the land sections.
Specifically, the respective focus offset amounts for the land sections and the groove sections are stored. Then, when tracking the groove sections, the focus servo is carried out by compensating the focus error signal based on the focus offset amount set for the groove sections. On the other hand, when tracking the land sections, the focus servo is carried out by compensating the focus error signal based on the focus offset amounts set for the land sections. As a result, the information can be recorded/reproduced in the tracking area of the optical disk under just-in-focus conditions.
The focus offset amount may be set by the following methods {circle around (1)} and {circle around (2)}.
Method {circle around (1)}: The focus offset amount is set based on the crosstalk between error signals at a time of assembling the optical pickup.
As described earlier, an optimal focal position differs between the land sections and the groove sections for the crosstalk between error signals of the optical pickup. Therefore, using the reference disk, by the crosstalk between the error signals obtained at a time of assembling the optical pickup, the respective optimal focal positions for the land sections and the groove sections are determined.
The method {circle around (2)}: the focus offset amount is set based on a reproducing signal obtained at a time of starting up an optical disk device.
After activating the optical disk device, before carrying out recording/reproducing information, a reproducing of signal (test reading) is carried out with variable focus offset, and the respective focus offset amounts for the land sections and the groove sections which maximize the resulting reproducing signal are calculated respectively.
The described method {circle around (1)} of setting the focus offset amount based on the error signal crosstalk obtained at a time of assembling the optical pickup and the method {circle around (2)} of setting the focus offset amount based on a reproducing signal generated when starting the optical disk device have the following problems.
When adopting the method {circle around (1)}, in an event that an amount of crosstalk between error signals varies due to changes over time of the optical pickup, an appropriate focus offset amount cannot be obtained, thereby presenting the problem that an optimal focus servo cannot be carried out. Furthermore, in this method, as a reference disk is used, if there exist variations between the reference disk and optical disks to be actually used, the above error occurs, resulting in the problem that an optimal focus servo cannot be carried out.
According to the method {circle around (2)}, although the problem associated with an error occurred when adopting the method {circle around (1)} does not occur, the following problem may arise. That is, for the test reading, it is required to change the focus offset amount, and therefore an additional time is required for the test reading at a time of starting up the device. Additionally, since it is required to carry out a system control for carrying out a test reading with variable focus offset amount, a cost of the device increases.
Japanese Unexamined Patent Publication No. 30975/1996 (Tokukaihei 8-30975) discloses a method of setting an offset amount which offers an optimal reproducing signal (signal amplitude or error rate) by carrying out a test reading with variable focus offset amount after starting up an optical disk device before carrying out recording/reproducing of information as in the described method {circle around (2)}. The method of this citation also raise the problems associated with the method {circle around (2)}.
It is an object of the present invention to provide an optical information recording/reproducing device which permits recording/reproducing information on and from a tracking area of an optical disk under just-in-focus conditions without increasing a cost of the device nor being affected by an error due to changes over time or variations in optical disks.
In order to achieve the above object, an optical information recording/reproducing device for recording/reproducing information by converging light on a recording medium including land sections and groove sections and detecting a light reflected therefrom using an optical pickup is characterized by including:
offset amount detection means for detecting a focus offset amount based on variations in amount of a focus error signal at a time a light beam crosses a boundary between a land section and a groove section adjacent to the land section, and
focus error signal compensation means for obtaining respective optimal focal positions for the land sections and the groove sections by compensating the focus error signal based on the focus offset amount as detected by the offset amount detection means.
According to the described arrangement, the offset amount detection means detects a focus offset amount based on variations in amount of a focus error signal obtained at a time a light spot crosses a boundary between the land section and the groove section adjacent to the land section. The focus error signal in the ON state of the focus servo varies as being affected by the crosstalk between error signals, and thus respective focus offset amounts of the land sections and the groove sections can be detected based on variations in amount of the focus error signal. Namely, in the state where there exits an eccentricity of the tracks formed by the land sections and the groove sections on the optical disk, only the focus servo is set ON, and respective focus offset amounts can be detected at a zero-cross timing of the tracking error signal obtained at a time the light spot crosses the track.
Based on the focus offset amount as detected by the offset amount detection means, the compensation means compensates a focus error signal. Therefore, recording and reproducing of information can be carried out in an optimal focal position at just-in-focus with respect to both the land sections and the groove sections, thereby obtaining a quality reproducing signal.
Since the described detection of the focus offset amount can be carried out without difficulties within the normal time required for starting up the device, the problems associated with the conventional device that a cost of the device increases, or a longer time is required to start up the device can be eliminated. Additionally, since a focus error signal can be compensated by detecting a focus offset amount for each optical disk, unlike the conventional arrangement of setting the focus offset amount based on the crosstalk between error signals obtained at a time of assembling an optical pickup using a reference disk, problems caused by errors due to variations over time and variations of optical disks can be avoided.
The described optical recording/reproducing device may be arranged so as to further include a comparator for inputting thereto a tracking error signal when the light spot passes a boundary between the land section and the groove section adjacent to the land section and sampling means for sampling a focus error signal in the ON state of a focus servo based on an output of the comparator.
In the described arrangement, it may be further arranged such that the comparator outputs a signal whose level varies at a center of each land section as a result of comparison with a predetermined reference level, and the sampling means samples a focus error signal when varying an output signal from said comparator.
According to the described arrangement, the comparator outputs a signal whose level varies at a center of each land section as a result of comparison with a predetermined reference level, and the sampling means samples a focus error signal when varying an output signal from said comparator.
To the comparator, input is an tracking error signal obtained when the light spot passes a boundary between a land section and the groove section adjacent to the land section. With an appropriate selection for the reference level, the output level can be varied at respective centers of the land section and the groove section. The sampling means can obtain the respective focus offset amounts for the land sections and the groove sections by sampling amounts of variations at respective centers of the land sections and the groove sections by sampling the focus error signal at a time the level of the output signal from the comparator varies.
The optical information recording/reproducing device of the present invention may be arranged so as to further include the second signal generation means for generating a second signal of the focus error signal which has a phase difference from a tracking error signal of 90xc2x0, the second signal showing its peak values at the center of each land section and the center of each groove section in the ON state of the focus servo at a time the light spot crosses the boundary between the land section and the groove section adjacent to the groove section, and by sampling the peak and the bottom of the envelope of the focus error signal, the respective focus offset amounts of the land section and the groove section can be detected.
As being affected by the crosstalk between the error signals, the focus error signal may vary with a phase difference of 90xc2x0 from the tracking error signal. Therefore, the focus error signal shows a peak value and a bottom value at a zerocross timing of the tracking error signal. Then, by sampling the peak and the bottom by detecting the envelope of the focus error signal, the focus offset amounts of the land section and the groove section can be detected.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.